Handle concurrent pmap_kextract() and promotion/demotion on arm64

[andrew]

arm64/arm64/pmap.c

1131–1132

This else is unneeded as the if will return.

[alc]

Deindent most of pmap_kextract().

Revise the comment in pmap_kextract() to refer to pmap_update_entry() rather than demotion and promotion because we also use pmap_update_entry() on L3 entries in pmap_change_attr_locked().